How do you use your iPhone as a usb key?

Solution 1:

I haven't tried it yet, but from this description, it looks like Air Sharing is what I want.

Solution 2:

Also try Dropbox which is useful even just on Macs no iOS devices. If file is put into a public share then you could see it on your PC as well not just his.

Solution 3:

Not 100% relevant to the OP, but given the prices of flash memory these days and the hurdles one has to jump through to transfer arbitrary data to/from iOS devices I wouldn't recommend it.

If the user instead wants to access/modify the data while it's on the device the practice makes sense, but otherwise it is much more convenient today to just have a USB thumbdrive handy.

(BTW: The app in question was indeed likely to be Air Sharing, an app that's been available almost as long as the SDK has.)